Apparent dipping activity early in the 2011 outburst of Aql X-1

ATel #4014; Duncan K. Galloway (Monash Centre for Astrophysics)
on 4 Apr 2012; 14:43 UT
Credential Certification: Duncan K. Galloway (duncan@space.mit.edu)

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I report on Rossi X-ray Timing Explorer (RXTE) observations of the low-mass X-ray binary Aql X-1, made on 2011 October 21 (obsid 96440-01-02-02), eight days after the detection of a new outburst (ATel #3686). Beginning on October 21.579 UT, I observed a series of four dips in X-ray intensity lasting between 5-60s each (see Fig. 1 in the attached link). The first two dips were separated by 3.9 min; then 7.8 min to the next (which occurred as a closely separated pair), and 9 min to the last. The observation ended 3 min later. During the second (and deepest) dip, the count rate was reduced to less than 20% of the level before or after; the other dips reached between 38-67% of the count rate before or after. One proportional counter unit (PCU #2) was operating, and the background-subtracted 2-60 keV count rate outside the dips was typically 470 counts/s.

The reduction of intensity during the dips was greatest at low energies, resulting in a hardening of the X-ray spectrum. The soft and hard X-ray colours (defined by the ratio of 16-s binned counts in the 3.7-4.9 keV and 2-3.7 keV bands, and the 8.6-17.7 keV and 4.9-8.6 keV bands, respectively) were significantly higher compared to the non-dip data. During the second (and deepest) dip, the soft (hard) colour reached 4.0 (1.39), compared to 1.3 (0.8) outside the dips (see Fig. 2). The typical maxima reached during normal activity of Aql X-1 were 1.65 (0.85; e.g. Galloway et al. 2008, ApJS 179, 360). The fitted neutral column density during the nondip intervals was 2.0e22 cm^{-2}; the X-ray spectrum during the second dip was consistent with an order-of-magnitude higher value, of (2.8+/-1.4)e23 cm^{-2}. Also required by the fit was a strong (tau=0.4) absorption edge at (5.2+/-0.5) keV.

The dips observed in Aql X-1 on October 21 bear a striking resemblance to the periodic dips observed in a handful of low-mass X-ray binaries, and generally attributed to obscuration of the neutron star by a thickened region of the accretion disk close to the line joining the two centres of mass (e.g. Diaz Trigo et al., Proc. HRXS conference, 2006). However, such features have not been previously reported in Aql X-1. To test the connection with the orbit, I compared the centroid time of the deepest (2nd) dip with the predicted time of optical minimum light (identified with inferior conjunction) for Aql X-1, based on the ephemeris for the 18.95-hr orbit adopted by Welsh et al. (2000, ApJ 120, 943). The dip centroid fell 880 s prior to the predicted epoch of minimum light, well within the 0.07-d projected 1-sigma uncertainty range (given the quoted precision in the orbital parameters). The probability of such an agreement by chance is approximately 2.6%.

While earlier RXTE observations of Aql X-1 have covered other inferior conjunctions, no similar dipping behaviour has been observed. That apparent dipping behaviour can occur at times suggests that the system inclination is at the high end of its likely range (36°-55°; e.g. Robinson et al. 2001, AIP Conf. Proc. 599, 902). On the other hand, the serendipitous observation of dips in this instance may be related to changes in the disk structure resulting from a spectral state transition during the outburst rise. In the subsequent RXTE observations, from October 24 onwards, the overall intensity was significantly higher, with soft colour remaining between 1.2-1.3, but the hard colour decreasing to between 0.32-0.4. This increase in intensity coupled with spectral softening likely indicates a transition from the "island" (hard) spectral state to the "banana" (soft) state some time between October 21 and 24.

I note that a single dip has been observed previously in one other non-dipping source, 3A 1820-30, in 1977 (Cominsky, Simmons & Bowyer 1985, ApJ 298, 581).

Supporting figures